This proposal has as its objective an understanding of how the cells in the immune system achieve self-tolerance during their development. The experimental model is that of tetraparental mice, rendered chimeric at the 8-cell stage by combining two embryos derived from different strains. We have already demonstrated that these mice as adults possess immune cells capable of destroying their own target tissues in culture, but at the same time have serum blocking factors, which can specifically prevent this destruction in vitro. This proposal outlines ways of determining the validity of this concept in vivo, as well as methods to chemically characterize the blocking factor. Further in vitro work should allow the determination of whether the blocking factor is made by T cells or by B cells. In order to see whether this concept of immune cells and blocking factors as mediators of tolerance applies to normal bi-parental individuals, a number of auto-immune models will be explored using the mixed lymphocyte culture technique. One of them, which involves the ability of CBA mouse newborn thymus cells to attack CBA adult spleen cells in vitro, will be explored to see if the mechanism of preventing this attack in the animal is serum blocking factor.